Traditionally, cast iron parts are cast into a desired shape, and then machined until proper tolerances are met. Some of these cast parts require hardening or tempering in order to prolong the lifetime of the part. In the past, the entire part may have been tempered by fully tempering or by through tempering to add wear resistance. For the manufacturer of parts, these wear enhancing methods are disadvantageous because they distort the shape of the part and make machining very difficult. Tempering a part includes heating the part to an austentizing temperature, and then quenching to cause compressive stresses to the material which hardens the part surface to add to its wearability. This has been very expensive due to high cycle times, high energy consumption, and the formation of low quality parts due to sagging during the heating stages.
Consequently, some historical attempts have been made to harden merely the surface of the part while allowing the cast core to remain untransformed. Flame hardening and induction have been used to locally heat the area before quenching to achieve surface hardening. These methods are limited in that they are not useful for cast parts which have many protrusions and indentations because flame hardening and induction methods cannot uniformly and perpendicularly heat all the surfaces at the same time. Manufacturers utilizing the surfaces at the same time. Manufacturers utilizing these methods have experienced problems due to uneven heating, non-homogeneous brittleness and low yield production. Furthermore, the surface hardened parts produced by such prior art methods are inherently expensive and difficult to manufacture and machine.
Conventional surface hardened methods are limited in that they are not useful for preparing mechanical parts which do not exhibit any distortion during heat treatment in order to provide the surface hardening. The previous methods have been unable to produce such articles at a low cost because their methods do not include the isopressure advantage which can conveniently, quickly and uniformly surface harden an entire mechanical part.
Previously, surface hardening was done by remelting the surface by irradiating a high density energy, for example, a TIG arc, and thereafter forming a continuous chill layer by self cooling. Other methods include pouring molten metal into a mold having a chill set therein, forming carbide in contact with the chill. Other methods have included inductively heating a preheated part to an austenizing temperature which was followed by quenching an isothermal transformation. The TIG torches mentioned above only cover a very small localized area, and it would be very advantageous to provide a method for uniformly and selectively surface hardening an entire part. TIG torches, or TIG arcs, only selectively heat a very small portion on the order of several millimeters in diameter. Consequently, in order to do a complete part such as a cam or gear surfaces, the prior art processes have been very labor and time intensive.
Yet still some other prior art methods for selectively hardening parts have included a strip hardening process and a combination induction heating through selectively austenizing a surface zone and thereafter quenching to cause a selectively higher dilatation in the surface zone. These methods also result in very small area surface hardening. And yet one more method for selectively surface hardening in the prior art includes an electron bombardment metal melting process which forms molten pools on the surface, followed by rapidly cooling the molten pool by the chilling effect of the non-molten portion of the cast iron part. Attempts to produce cast iron parts having surface hardening over the entire surface have met with failure because these prior art methods have all been limited to the local surface areas which are covered by the surface hardening technique.
Examples of previous attempts to increase production while preparing a cast iron part having a hardened surface are described in the following patents:
U.S. Pat. No. 4,720,321 issued to Toyota on Jan. 19, 1988 discloses a process for producing surface remelted chilled layer cam shafts. The method includes the step of melting a sliding cam surface by subjecting to high density energy through a TIG arc and thereafter forming a chilled layer by self cooling.
U.S. Pat. No. 4,772,340 issued to Honda Motor Company on Sep. 20, 1988 discloses a method of making iron-based articles having a remelted layer which is formed by pouring molten metal into a mold which has a chill set therein to cast iron based articles with chilled regions formed that have come into contact with the chilled set.
U.S. Pat. No. 4,312,685 issued to Audi Motor Company on Jan. 26, 1982 discloses a surface hardening method for cams of motor vehicle cam shafts. The cam has its surface hardened by rotating the cam shaft about its axis while maintaining a TIG torch at a fixed spacing from the surface, and while relatively axially reciprocating the torch and the cam shaft so that the torch heats the surface at an undulating path. After removing the TIG torch, the remelted layer then chills to provide a surface hardened area.
U.S. Pat. No. 4,643,079 issued to General Motors Corporation on Feb. 17, 1987 discloses an iron piston having selectively hardened ring grooves. In the preferred embodiment, the groove faces were hardened by a strip hardening process.
U.S. Pat. No. 3,477,884 issued to Schlicht, et al. on Nov. 11, 1969 discloses a method of increasing the fatigue life of rolling contact elements which is accomplished by a combination of induction heating to selectively austenitize a surface zone adjacent the rolling contact surface, quenching to cause a selectively higher dilatation in the surface zone, and inducting tempering of the quenched element so as to produce in the surface zone a volume contraction which provides a residual compressive stress on the surface.
U.S. Pat. No. 4,000,011 issued to Sato et al. on Dec. 28, 1976 discloses a method of surface hardening which includes the steps of rapidly melting a local surface by means of an electron bombardment melting process to form a molten pool thereon. Thereafter, the molten pool is rapidly cooled by the chilling effect of the non-molten portion of the cast iron. The cast iron part having the hardened layer is finally finished to form a desired shape.
Therefore, it is a primary object of the present invention to provide a selective surface hardening method in accordance with the present invention which will produce cast iron parts which may be easily, inexpensively, and uniformly surface hardened over the entire surface of the cast iron part.
It is another object of the present invention to provide a method which will yield cast iron parts which may be pre-finish-machined before the surface hardening step of the method is accomplished. Thereby, cast iron parts may be placed in their final form and machined while the surface is easy to machine, and may then be selectively surface hardened thereafter without having to do substantial finish machining on a hardened part.
It is yet another object of the present invention to provide a method which will produce a selectively surface hardened cast iron part which may be machined before surface hardening while not producing any substantial size distortion after the finish machining has been accomplished.